1.写在前面的废话
前面几次关于 IOC 的分享完成了对 Spring 对资源的加载的一个概述。读者可以对 Spring 加载资源有一个大致的了解。今天我们将开始了解当资源加载到 Spring 容器中后，如何进行依赖注入的。
学习 Spring，有一个非常重要的概念就是控制反转，控制反转就是要把资源装配工作丢给 Spring 容器，业务只需要知道自己需要什么样的服务就可以了。Spring 装配 Bean 的依赖是一个比较复杂的过程，基本上是横看成岭侧成峰，远近高低各不同的状态。今天我们从一个基本了解的角度大概了解一下依赖注入的整个过程。
2.依赖注入
2.1依赖注入从何处开始
我们知道 Spring 可以是懒加载的，就是当真正使用到 Bean 的时候才实例化 Bean。当然也不全是这样，例如配置 Bean 的 lazy-init 属性，可以控制 Spring 的加载时机。现在机器的性能、内存等都比较高，基本上也不使用懒加载，在容器启动时候来加载bean，启动时间稍微长一点儿，这样在实际获取 bean 供业务使用时，就可以减轻不少负担，这个后面再做分析。 我们使用到 Bean 的时候，最直接的方式就是从 Factroy 中获取，这个就是加载 Bean 实例的源头。
从了解 AbstractBeanFactory 开始：
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- AbstractBeanFactory 的位置
AbstractBeanFactory 是大部分 BeanFactory 的基类。他有两方面的能力，一方面是 Bean 注册的能力，另一个方面是 BeanFactory 的固有服务的能力。前者可以判断注册的 Bean 是否是单例、别名注册等；后者让客户端可以获取 Bean 实例。
- AbstractBeanFactory 中的 getBean 方法
// 我们经常使用的方法，通过 Bean 名称获取 Bean 实例
public Object getBean(String name) throws BeansException {
// 交给 doGetBean 处理
return doGetBean(name, null, null, false);
}
// 通过指定的 Bean 名称和获取 Bean 的类型，获取 Bean 实例
public <T> T getBean(String name, Class<T> requiredType) throws BeansException {
// 交给 doGetBean 处理
return doGetBean(name, requiredType, null, false);
}
// 通过 Bean 名称和指定构造 Bean 实例的工厂方法参数来获取 Bean 实例
public Object getBean(String name, Object... args) throws BeansException {
// 交给 doGetBean 处理
return doGetBean(name, null, args, false);
}
/**
* Return an instance, which may be shared or independent, of the specified bean.
* @param name the name of the bean to retrieve
* @param requiredType the required type of the bean to retrieve
* @param args arguments to use if creating a prototype using explicit arguments to a
* static factory method. It is invalid to use a non-null args value in any other case.
* @return an instance of the bean
* @throws BeansException if the bean could not be created
*/
public <T> T getBean(String name, Class<T> requiredType, Object... args) throws BeansException {
// 交给 doGetBean 处理
return doGetBean(name, requiredType, args, false);
}
/**
* Return an instance, which may be shared or independent, of the specified bean.
* @param name the name of the bean to retrieve
* @param requiredType the required type of the bean to retrieve
* @param args arguments to use if creating a prototype using explicit arguments to a
* static factory method. It is invalid to use a non-null args value in any other case.
* @param typeCheckOnly whether the instance is obtained for a type check,
* not for actual use
* @return an instance of the bean
* @throws BeansException if the bean could not be created
*/
@SuppressWarnings("unchecked")
protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
// 处理别名，找出真正的 Bean 名称
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isDebugEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
}
}
// 处理 FactoryBean 的方式的实例
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
// 如果正在创建 Bean 跳出处理
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
// 如果父容器不为空，并且本容器中没有对应的 BeanDefinition 时，交给父容器递归处理。
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
if (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
// 获取 BeanDefinition 定义
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
// 查找 BeanDefinition 中依赖，递归处理实例化依赖的 Bean。
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dependsOnBean : dependsOn) {
getBean(dependsOnBean);
registerDependentBean(dependsOnBean, beanName);
}
}
// Create bean instance.
// 默认就是单例模式
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
public Object getObject() throws BeansException {
try {
// createBean 是虚方法，等待子类实现
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
// Explicitly remove instance from singleton cache: It might have been put there
// eagerly by the creation process, to allow for circular reference resolution.
// Also remove any beans that received a temporary reference to the bean.
destroySingleton(beanName);
throw ex;
}
}
});
// 处理 FactoryBean
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
// 如果是多例模式
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
// 如果是自定义的范围类型
else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
public Object getObject() throws BeansException {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
}
});
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new BeanCreationException(beanName,
"Scope '" + scopeName + "' is not active for the current thread; " +
"consider defining a scoped proxy for this bean if you intend to refer to it from a singleton",
ex);
}
}
}
// Check if required type matches the type of the actual bean instance.
if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
try {
return getTypeConverter().convertIfNecessary(bean, requiredType);
}
catch (TypeMismatchException ex) {
if (logger.isDebugEnabled()) {
logger.debug("Failed to convert bean '" + name + "' to required type [" +
ClassUtils.getQualifiedName(requiredType) + "]", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}
2.2 DefaultSingletonBeanRegistry 中的 getSingleton 方法
在 2.1 中我们知道在 getBean 中有调用 getSingleton 方法，从而来回调来调用 createBean 方法。
下面是 getSingleton 的源码：
public Object getSingleton(String beanName, ObjectFactory singletonFactory) {
Assert.notNull(beanName, "'beanName' must not be null");
// 同步 singletonObjects 集合
synchronized (this.singletonObjects) {
// 先获取一遍，如果可以获取到直接返回实例
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while the singletons of this factory are in destruction " +
"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
if (logger.isDebugEnabled()) {
logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
}
beforeSingletonCreation(beanName);
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<Exception>();
}
try {
// 回调创建单例实例的策略方法创建 Bean 实例
singletonObject = singletonFactory.getObject();
}
catch (BeanCreationException ex) {
if (recordSuppressedExceptions) {
for (Exception suppressedException : this.suppressedExceptions) {
ex.addRelatedCause(suppressedException);
}
}
throw ex;
}
finally {
if (recordSuppressedExceptions) {
this.suppressedExceptions = null;
}
afterSingletonCreation(beanName);
}
addSingleton(beanName, singletonObject);
}
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
}
2.3创建 Bean 实例
创建 Bean 实例，我们必须讲到AbstractAutowireCapableBeanFactory。他继承于 AbstractBeanFactory，子类有 DefaultListableBeanFactory。基本上所有的 ApplicationContext 实例都会实现 DefaultListableBeanFactory，所以这个方法应该是容器里面核心方法。
源码如下：
/**
* Central method of this class: creates a bean instance,
* populates the bean instance, applies post-processors, etc.
* @see #doCreateBean
*/
@Override
protected Object createBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
throws BeanCreationException {
if (logger.isDebugEnabled()) {
logger.debug("Creating instance of bean '" + beanName + "'");
}
// Make sure bean class is actually resolved at this point.
// 确保 BeanClass 真正存在
resolveBeanClass(mbd, beanName);
// Prepare method overrides.
try {
mbd.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbd.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
// 定义自己 BeanPostProceccor 来创建 Bean 实例的代理实现，如果创建成功直接返回
Object bean = resolveBeforeInstantiation(beanName, mbd);
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
}
// 如果没有用户自定义的创建 Bean 实例的代理，那么执行通用的创建 Bean 实例的方法
Object beanInstance = doCreateBean(beanName, mbd, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
/**
* Actually create the specified bean. Pre-creation processing has already happened
* at this point, e.g. checking <code>postProcessBeforeInstantiation</code> callbacks.
* <p>Differentiates between default bean instantiation, use of a
* factory method, and autowiring a constructor.
* @param beanName the name of the bean
* @param mbd the merged bean definition for the bean
* @param args arguments to use if creating a prototype using explicit arguments to a
* static factory method. This parameter must be <code>null</code> except in this case.
* @return a new instance of the bean
* @throws BeanCreationException if the bean could not be created
* @see #instantiateBean
* @see #instantiateUsingFactoryMethod
* @see #autowireConstructor
*/
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
Class beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
mbd.postProcessed = true;
}
}
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
// 关于在创建 Bean 实例前需要的定义执行的 BeanPostProcessor 处理
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isDebugEnabled()) {
logger.debug("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
addSingletonFactory(beanName, new ObjectFactory() {
public Object getObject() throws BeansException {
return getEarlyBeanReference(beanName, mbd, bean);
}
});
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
// 组装 Bean
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
}
catch (Throwable ex) {
if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
throw (BeanCreationException) ex;
}
else {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
}
}
// 二次验证在 Bean 创建前处理的 BeanPostProcessor 是否正确
if (earlySingletonExposure) {
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
}
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
for (String dependentBean : dependentBeans) {
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
if (!actualDependentBeans.isEmpty()) {
throw new BeanCurrentlyInCreationException(beanName,
"Bean with name '" + beanName + "' has been injected into other beans [" +
StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
"] in its raw version as part of a circular reference, but has eventually been " +
"wrapped. This means that said other beans do not use the final version of the " +
"bean. This is often the result of over-eager type matching - consider using " +
"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
}
}
}
}
// Register bean as disposable.
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
3.小结
本次分享主要了解了依赖注入的入口，以及依赖注入的大概过程。Bean 在做依赖注入时，通过我们常用的 getBean 方法开始，最终到AbstractAutowireCapableBeanFactory.doCreateBean 方法，整个模板方法还是相对比较简单，功能也比较内聚。过程中我们注意到有处理 FactroyBean 的入口、处理单例多例模式的入口、扩展点（BeanPostProcessor）的入口、组装 Bean 实例的入口等，这些都可能我们调查问题、扩展框架的源头。在创建完 Bean 以后，这个 bean 实例就可以真正的被业务所使用。依赖注入的过程是一个相对比较复杂过程，也是 Spring 的核心，很难在一次分享中就能介绍的全面，所以下面的几次分享中，我们将从以下几个方面分角度了解：
- Bean 实例化过程
- Bean 自动装载的过程
- Bean 中扩展点（BeanPostProcessor）的处理过程
- Bean 中scope处理过程（特别是单例的处理过程）